Display device

ABSTRACT

In a display device comprising a first substrate having at least one transparent, first picture electrode of a first material, a second substrate comprising at least one second picture electrode of a second material which, jointly with the picture electrode on the first substrate and an intermediate opto-electronic material, defines a pixel, and means for supplying electric voltages to the picture electrodes, the work function between the two picture electrodes is decreased in that at least one of the picture electrodes is coated with at least one layer of conducting material, or a layer of a material comprising a dipole.

[0001] The invention relates to a display device comprising a firstsubstrate having at least one transparent, first picture electrode of afirst material, a second substrate having at least one second pictureelectrode of a second material which, jointly with the picture electrodeon the first substrate and an intermediate opto-electronic material,defines a pixel, and means for supplying electric voltages to thepicture electrodes.

[0002] The invention also relates to a display device comprising a firstsubstrate having at least one transparent, first picture electrode of afirst material, a second substrate having a channel plate coated with adielectric layer, in which a channel provided with channel electrodes,jointly with the picture electrode on the first substrate and anintermediate opto-electronic material, defines a pixel, and means fordriving the pixel.

[0003] Display devices of this type are generally known and usuallycomprise a large number of pixels. The first type comprises, forexample, LCDs, both of the active and the passive type; the second typecomprises plasma-addressed liquid crystal display devices referred to asPALC displays.

[0004] Notably when the second picture electrode is reflective, thedrive of these display devices appears to be sensitive to thealternating supply of positive and negative voltages across the pixels.This becomes manifest as, for example, flicker. In a picture period, inwhich a positive voltage is written, the same picture information leadsto a different voltage across the pixel than in the subsequent pictureperiod in which a negative voltage is written. At a frame frequency of,for example 60 Hz, this leads to a flicker frequency of 30 Hz, which isclearly noticeable in the picture. The display devices also often sufferfrom image retention. In this application, reflecting picture electrodesare also understood to mean partially reflecting, semitransparentelectrodes as are used in transflective display devices.

[0005] It is an object of the invention to partly or entirely obviateone or more of said problems.

[0006] A first embodiment of a display device according to the inventionis therefore characterized in that at least one of the pictureelectrodes is coated with at least one layer of conducting material,such that the difference in work function between the two pictureelectrodes is decreased.

[0007] A second embodiment of a display device according to theinvention is characterized in that at least one of the pictureelectrodes is coated with at least one layer of a material comprising adipole, such that the difference in work function between the twopicture electrodes is decreased.

[0008] The embodiment of a display device according to the invention,based on plasma addressing, is characterized in that the pictureelectrode, the dielectric layer or at least one channel electrode iscoated with at least one layer of material modifying the work function.

[0009] The invention is based on the recognition that saidimage-disturbing phenomena (flicker) result from an initial internal DCvoltage across the electro-optical layer. In its turn, this voltagegives rise to ion movements which lead to image retention or cause saidtransient effect.

[0010] Said voltage is presumably the result of the difference in workfunction between the materials of the two picture electrodes, forexample ITO for the transparent picture electrode and aluminum for thereflective picture electrode. Such an asymmetrical structure thus leadsto an internal DC voltage which, according to the invention, isneutralized by coating one of the picture electrodes with a layer ofconducting material or a layer of material comprising a dipole, forexample, an organic material having a low electric conductance, suchthat the difference in work function between the two picture electrodesis decreased. Experiments have proved that the difference in workfunction, dependent on the materials used, can be reduced to less than0.25 V.

[0011] Since also PALC display devices have an asymmetrical structure (aliquid crystal layer between a microsheet and an electrode) there isalso such an internal DC voltage in this case which, according to theinvention, is neutralized by coating the picture electrodes, themicrosheet or the channel electrodes with at least one layer of materialmodifying the work function of the material of said picture electrodes,microsheet, or channel electrodes.

[0012] The difference in work function may be decreased in differentways, for example, by coating the transparent picture electrode with alayer comprising at least a thin layer of the material of the reflectingpicture electrode, or by coating the reflecting picture electrode with alayer comprising at least a layer of conducting material havingsubstantially the same work function as that of the transparent pictureelectrode. An example of the latter case is the use of a thin layer ofgold on a silicon wafer on which, for example, reflecting aluminumelectrodes are realized, while switching elements (transistors) arerealized in the subjacent silicon. If the gold is several monolayersthick, there is hardly any lateral conductance and this layer does notneed to be split up into separate pixels.

[0013] The reflecting picture electrode as well as the transparentpicture electrode may be coated with a layer comprising at least a layerof the same conducting material.

[0014] Also when using (organic) material with a dipole, which ispreferably chosen from the group of polyimides or polyamide acidscomprising or not comprising fluorine, the reflecting picture electrodeand the transparent picture electrode may be coated with a layercomprising at least a layer of the same (organic) material with adipole, the material on one of the two picture electrodes having beensubjected to, for example, an UV treatment when these electrodes arecoated with a layer comprising at least a layer of the same (organic)material with a dipole.

[0015] If desired, the layer of material with a dipole may function as aorientation layer.

[0016] These and other aspects of the invention are apparent from andwill be elucidated with reference to the embodiments describedhereinafter.

[0017] In the drawings:

[0018]FIG. 1 is a cross-section of a part of a display device accordingto the invention,

[0019]FIGS. 2, 3, 4, 5 and 6 are variants of FIG. 1.

[0020] The Figures are diagrammatic and not to scale. Correspondingcomponents generally have the same reference numerals.

[0021]FIG. 1 is a cross-section of a part of a display device comprisingan electro-optical display cell, in this example a liquid crystal cell 1with a twisted nematic liquid crystal material 2 present between a firsttransparent substrate 3 of, for example, glass comprising an electrode5, and a second substrate 4. The electrode 5 is made of alight-transmissive material, for example indium tin oxide (ITO). Thesecond substrate 4 comprises electrodes 6 of a reflecting or diffusereflecting material such as, for example aluminum. The second substrate4 is opaque in this example and may consist of various materials, forexample, a silicon wafer in which switching elements are integrated.

[0022] Different electro-optical effects may be used, particularlyliquid crystal effects such as (S)TN, guest-host, PDLC, ferro-electrics,VAN, reflective OCB, HAN. Dependent on the effect used, the devicecomprises a polarizer 7. In this example, the device also comprisesorientation layers 8, 9 which orient the liquid crystal material on theinner walls of the substrates, such that the cell has a twist angle ofapproximately 90° in this example. The display device comprises drivemeans for supplying drive voltages to the electrodes 5, 6. These drivemeans are diagrammatically indicated by means of the switch 10 and avoltage source 21 in FIG. 1. In this first example, the ITO layer 5 iscoated with a second metal layer 12 of aluminum.

[0023] The work function, both with and without the metal layer 12, wasmeasured of the first substrate 3 coated with an ITO layer 5 and anorientation layer 8. For such a measurement (the Kelvin method), avibrating gold plate is placed opposite the substrate at a smalldistance (0.1 mm), while an electric voltage is supplied between theplate and the ITO electrode. The vibration of the plate causes achanging capacitance, which gives rise to a current which is eliminatedby means of a counter voltage which is a measure of the work function.Similarly, the work function was measured of the second substrate 4coated with an aluminum pattern 6 and an orientation layer 9. Due to thepresence of the metal layer 12, the difference in work function of thesubstrates 3, 4 had decreased considerably.

[0024] Subsequently, the flicker was measured of the finished cellfilled with liquid crystal material. This can be done optically byapplying a square-wave voltage with an identical positive and negativeamplitude across a pixel, by measuring the flicker (for example, bymeans of an exposure meter) and by giving such an offset that theflicker is substantially invisible. Another method is purely electricalbut only suitable for active drive. In this method, the voltagevariation during non-selection across a pixel is fixed by means ofsample-and-hold circuits. Voltage differences which occur during anegative and a subsequent positive field or frame are mutually comparedwhereafter, if necessary, an offset voltage is introduced to eliminatethe voltage differences. It was found that the required offset voltagefor avoiding flicker had substantially the same value as the differencein work function of the substrates 3, 4. By minimizing this difference,in this case by providing the aluminum layer 12, the internal DC voltage(and hence flicker, but also image retention and said transientphenomena) are thus reduced considerably.

[0025] With a view to transparency, the aluminum should not be too thick(several monolayers). However, thin layers of aluminum tend to reactwith the ITO while forming aluminum oxide. This has a different workfunction and, moreover, the conductance is reduced. Another possibilityis shown in FIG. 2, in which the difference in work function of thesubstrates 3, 4 has decreased considerably by coating the aluminumelectrode with a second ITO layer 11. A thin layer of a less reactivematerial, such as silver, may be alternatively used as an intermediatelayer. For the sake of simplicity, the polarizer 7 and the switch 10have been omitted in FIGS. 2, 3, 4 and 5. The original potentialdifference (the DC offset voltage) of 0.8 V was reduced to <0.1 V insuch a device.

[0026] The metal layer 12 shown in FIG. 1 is replaced in another exampleby (or, if necessary, coated with) a metal layer whose material has asubstantially identical work function such as, for example, molybdenum,chromium, tungsten, silver or gold, if the metal layer 12 consists ofaluminum.

[0027] The transparent conductor 5 may be alternatively manufacturedfrom a different material than ITO but with a substantially identicalwork function, such as SnO/In₂O₃ or polyaniline.

[0028] In yet another example, the metal layer 12 consists of a thindouble layer. The ITO is coated with a thin layer of a metal which doesnot react with ITO such as, for example, molybdenum, chromium, tungstenor titanium, which layer in its turn is coated with a thin layer ofaluminum.

[0029] A further possibility is shown in FIG. 3, in which the differencein work function of the substrate 3, 4 has decreased considerably bycoating both the aluminum electrode 6 and the ITO electrode 5 with alayer 12 of the same metal. This metal is chosen from material whichpreferably does not react with ITO, for example, from the group ofmolybdenum, copper, tungsten and titanium; double layers arealternatively possible. The layer on the ITO must of course be thinenough to pass light.

[0030] In the device of FIG. 4, a silicon wafer is used for thesubstrate 4, in which switching elements energizing the switchingelectrodes 6 are realized. The switching electrodes are mutuallyseparated, while the entire wafer is coated with a metal layer 14 ofgold which is only several monolayers thick. By coating the surface withgold, the reflectivity is hardly influenced. Besides, the layer is sothin that no or hardly any lateral conductance occurs; the voltageacross an electrode 6 is thus entirely determined by a subjacentswitching element. Gold has substantially the same work function as ITOso that a symmetric reflective display cell is obtained which may beused notably in projection applications. Here again, the DC offsetvoltage was reduced from 0.8 V to values below 0.25 V.

[0031]FIG. 5 is a cross-section of a part of a display device comprisingan electro-optical display cell, in this example a liquid crystal cell 1with a twisted nematic liquid crystal material 2 which is presentbetween a first transparent substrate 3 of, for example glass comprisingan electrode 5, and a second substrate 4 comprising an electrode 6. Theelectrodes are coated on at least one substrate with layers 18, 19 of anorganic material increasing or decreasing the work function (forexample, hydrocarbons comprising or not comprising fluorine). Thesematerials have a small conductance and comprise an internal dipole. Ifdesired, the layers 18, 19 are coated with layers 8, 9 of orientingmaterial. If a polyamide acid material, a precursor polyimide, amaterial comprising fluorine, or (preferably) a polyamide acid materialcomprising fluorine is chosen, the work function of the aluminum isincreased to approximately that of the ITO. An example is a polyimide ofpyromeeletic dianhydride and2.2-bis[4-(4-aminophenoxy)phenyl]-hexafluoropropane.

[0032] A layer 18 of polyethyleneimine decreases the work function ofthe ITO. Both substrates may of course be coated with the same materialcomprising a dipole and multilayer structures are alternativelypossible. The layers comprising the dipole are obtained in thatcompounds which constitute a permanent dipole in combination with theelectrode material are used for the manufacture of said layers.

[0033] If necessary, the difference in work function is even furtherdecreased by subjecting the display device, or at least one of the twosubstrates, to an UV treatment. Generally, this slightly decreases thework function of ITO.

[0034] The invention is of course not limited to the examples shown. Forexample, in FIG. 5, the layers 18, 19 may be made of a material which issuitable as an orientation material for the liquid crystal material. Thelayers 8, 9 can then be dispensed with. The substrate 4 may alsoconstitute, for example a channel plate for a display device of thePlasma-Addressed Liquid Crystal type (PALC). In that case, as shown inFIG. 6, the electrodes 6 are dispensed with and the substrate 4 isprovided with elongated plasma ducts 16, covered by a microsheet 15. TheDC offset voltage is decreased by coating the electrode 5 on thetransparent substrate 3, the microsheet 15 or one or both electrodes 17in the channels 16 with a layer 20 modifying their work function.(Dependent on its position in the device (either on electrode 5, on themicrosheet or on a channel electrode) a material either decreasing orincreasing the work function is chosen.) Combinations are alternativelypossible.

[0035] As has been stated, the electrodes 6 may be transparent or have,for example a pattern of apertures for transflective display devices.

1. A display device comprising a first substrate having at least onetransparent, first picture electrode of a first material, a secondsubstrate comprising at least one second picture electrode of a secondmaterial which, jointly with the picture electrode on the firstsubstrate and an intermediate opto-electronic material, defines a pixel,and means for supplying electric voltages to the picture electrodes,characterized in that at least one of the picture electrodes is coatedwith at least one layer of conducting material, such that the differencein work function between the two picture electrodes is decreased.
 2. Adisplay device comprising a first substrate having at least onetransparent, first picture electrode of a first material, a secondsubstrate having at least one second picture electrode of a secondmaterial which, jointly with the picture electrode on the firstsubstrate and an intermediate opto-electronic material, defines a pixel,and means for supplying electric voltages to the picture electrodes,characterized in that at least one of the picture electrodes is coatedwith at least one layer of a material comprising a dipole, such that thedifference in work function between the two picture electrodes isdecreased.
 3. A display device as claimed in claim 1 or 2 ,characterized in that the second picture electrode is reflective.
 4. Adisplay device as claimed in claim 1 or 2 , characterized in that atleast one of the picture electrodes is coated with at least a layer ofconducting material and at least a layer of a material comprising adipole.
 5. A display device as claimed in claim 1 , 2 or 4,characterized in that the difference in work function between the twopicture electrodes is at most 0.25 V.
 6. A display device as claimed inclaim 1 , 2 or 4, characterized in that the first picture electrode iscoated with a layer comprising at least a thin layer of the material ofthe second picture electrode.
 7. A display device as claimed in claim 1, 2 or 4, characterized in that the second picture electrode is coatedwith a layer comprising at least a layer of conducting material havingsubstantially the same work function as that of the first pictureelectrode.
 8. A display device as claimed in claim 1 , 2 or 4,characterized in that the first picture electrode and the second pictureelectrode are coated with a layer comprising at least a layer of thesame conducting material.
 9. A display device as claimed in claim 2 or 4, characterized in that the material comprising a dipole is chosen fromthe group of polyimides and polyamide acids.
 10. A display device asclaimed in claim 9 , characterized in that the material having a dipolecomprises fluorine-containing polyimides or polyamide acids.
 11. Adisplay device as claimed in claim 2 or 4 , characterized in that thefirst picture electrode and the second picture electrode are coated witha layer comprising at least a layer of the same material having adipole.
 12. A display device as claimed in claim 2 or 4 , characterizedin that the first picture electrode and the second picture electrode areeach coated with a layer comprising a layer of different materialshaving a dipole.
 13. A display device as claimed in claim 2 or 4 ,characterized in that the first picture electrode and the second pictureelectrode are coated with a layer comprising at least a layer of thesame material having a dipole, the material on one of the two pictureelectrodes being subjected to an UV treatment.
 14. A display devicecomprising a first substrate having at least one transparent, firstpicture electrode of a first material, a second substrate having achannel plate coated with a dielectric layer, in which a channelprovided with channel electrodes, jointly with the picture electrode onthe first substrate and an intermediate opto-electronic material,defines a pixel, and means for driving the pixel, characterized in thatthe picture electrode, the dielectric layer or at least one channelelectrode is coated with at least one layer of material modifying thework function of the material of said picture electrode, dielectriclayer, or channel electrode.